Although the slowness in of the 260.xx drivers is being discussed in a few threads for quite a while, the fact that the issue is not resolved even with the most recent driver (260.19.29) seems to indicate that NVIDIA is not paying too much attention to it. Perhaps starting a new thread to explicitly discuss this issue might help. The basic flaw is the low decoding speed in comparison with the old driver, in particular 256.53, to which those people are stuck who are affected. From reading the threads, my impression is that the problem is often associated with 8400/8600 GPUs which might be a hint. To make the case clear (once more) here are (again) the numbers.